The present invention relates to the field of building or rehabilitation of housing or premises wherein persons live and/or equipment is stored, which have to be protected against electromagnetic waves, such as data processing or data storing centers, firms, offices, hospitals, dwellings. The building external envelope mainly consists of concrete, with a processing limiting the transmission of electromagnetic waves and forming an Electro Magnetic Protection (PEM). Such protection can be provided in two ways:
Protection against the intrusion of electromagnetic waves in premises;
Protection against the emission of electromagnetic waves, from the premises outwards.
For decades, protecting premises against radio-electric fields was required in very specific cases only: specialized measurements laboratories, premises on military sites with powerful transmitters, some premises in embassies or hospitals, etc.
From the end of the 90s on, the needs for a radio-electric protection of large premises (>30 m2) even full buildings have grown. They originate from a better knowledge of threats entailed in high frequency strong fields, on civilian or military sites. The so-called anti-compromise protection or electromagnetic spying protection.
Other threats have been added to these potential threats: modern radio-electric weapons, which generate very strong fields (>1,000V/m) as ultra-short pulses (ultra wide band pulses) or a highly-pinned microwave beam (Hi-Power MicroWaves). Technological progress made such weapons easy to use by conventional belligerents or terrorists.
State of the Art
A first protection mode is known in the state of the art, which consists in providing a Faraday cage, which surrounds the room or the building desired to be sealed or protected from electromagnetic waves.
When the external envelope of the premises mainly consists of concrete, the Faraday cage is, in most cases, a coating consisting of a metallic layer which is added to the wall of the premises desired to be protected. Such metallic layer, often made of copper, is relatively expensive.
A second solution consists in integrating metallic fillers into concrete in order to increase the absorption rate of concrete used for building the enclosure. Introducing metallic particles into concrete originally aims at modifying the mechanical behaviour characteristics of concrete, as explained in the article published in «Béton[s]» magazine, dated April-March 2013, and entitled «Les fibres métalliques» by Frédéric Cluzicki.
A third family of solutions, corresponding to the closest state of the art, consists in combining the use of a metallic layer, specifically as a grid, with the use of concrete containing metallic fillers.
For instance, the international patent application WO 2012114448 discloses a reinforced concrete building that exhibits a function of electromagnetic wave shielding against electromagnetic waves. This RC wall attenuates electromagnetic waves with the frequency to be shielded that propagate from the interior space (R1) toward the exterior space (R2). It consists of a concrete section the width direction of which coincides with the propagating direction from the interior space toward the exterior space, a front-side rebar grid on the interior space side of the concrete section, a rear-side rebar grid on the exterior space side, and a welded wire mesh that is provided between the front-side rebar grid and the rear-side rebar grid.
The distances between the rebar planes and the welded wire mesh in the propagation direction are adjusted to become an integral multiple of one wavelength of the frequency to be shielded, and the spacing within the front-side rebar grid and the rear-side rebar grid is adjusted to become 3 times as wide as the spacing within the welded wire mesh.
Such solution is adapted to the protection of radiation having a specific wavelength and does not enable a “wide-band” protection.
The Chinese patent CN 103903665 relates to a cement mortar wideband shielding/microwave absorbing composite structure building material with metal net and electromagnetic function. The metallic mesh is embedded in the cement mortar.
The Japanese patent JPH01302897 discloses another solution for a structure which can stop radio waves.
It provides to use materials as a similar grid, with high conductivity, connected and positioned inside the concrete containing a conductive element, i.e. as particles, powder or pellets mixed with concrete in a volume ratio ranging from 1 to 3%.
The shielding panel is formed by adding steel or similar particles to plain concrete. The core materials such as iron or similar rods or grids are embedded in concrete. Frame elements consisting of angle irons or similar are provided on a peripheral edge portion. Individual panels are connected together, whereas frame elements are assembled using coupling bolts or the like.
The international patent application WO2014/210007 discloses another exemplary solution of the prior art, where the metallic mesh is embedded in a volume of concrete containing metallic fibres. Paragraph [0006] of the above-mentioned patent application says «concrete structure formed using conductive concrete mixture and conductive screens.» and paragraph [0019] says «The conductive concrete mixture also includes one or more conductive materials configured to furnish electrical conductivity to the concrete. The conductive material serves to provide EMP shielding and reflect and absorb, for instance, EM waves propagating through the conductive concrete mixture. For example, the conductive concrete mixture may include at least substantially uniformly distributed conductive materials, which may include metallic and possibly non-metallic conductive materials, such as metal and/or carbon fibers.».
Another similar solution is provided in the Japanese patent JPH05222785, which relates to the execution of armouring on a wide range, from a low frequency band to a high frequency band, and reducing shielding operations.
Shielding is provided by an electromagnetic protection element during the installation in a portion of a concrete conductive wall.
Concrete consists in a mixture with a particle of carbon or the like, and a metallic mesh is integrated in the conductive concrete to provide electrical connection between the electromagnetic shielding element and the conductive concrete.
The American patent U.S. Pat. No. 5,908,584 is also known in the state of the art, and discloses an electrically conductive material used for shielding against electromagnetic waves and comprising a binder, a mixture of graphite and amorphous carbon, and sand.
The European patent application EP0745061 is also known, which discloses a conductive cement-based composition exhibiting both good electrical conductivity and mechanical strength with dispersion of conductive phase within the composition.
The wide ranges of contents in the conductive composition are as follows: a cement-based binder, a conductive phase consisting of one or more of the following elements: conductive fibres in an amount of 0 to 15% by volume of the composition; conductive particles in an amount of 0 to 80% by volume of the composition; water, with a ratio, by weight, to the cement binder of 0.2 to 0.75, fine aggregates with a ratio, by weight, to the cement binder of 0.0 to 2.0, and coarse aggregates with a ratio, by weight, to the cement binder of 0.0 to 2.0, conventional additives or mixtures and a dispersing agent, if need be.
Drawbacks of the Prior Art
When shielding large dimension premises, the solutions like a Faraday cage using a sealed coating of six faces with steel or copper sheets become very expensive as regards materials as well as labour, all the more so since they are implemented as light work, once the main building is elevated. Besides, the protected premises is always provided with passages for persons and most often with passages for fluids (water, air, high power, low power).
The third family solutions, which consist in embedding a metallic grid in concrete containing particles are not easily implemented: concrete is filled with layers, the density of which gradually increases in the wall thickness. Besides, such solutions are based on the partial absorption of electromagnetic waves by concrete containing conductive particles increased by the reflection on the metallic mesh embedded in such filled concrete.
The efficiency of absorption is however limited for thin walls, and the mesh embedded in the concrete core is less efficient than when laid on a surface.